The invention concerns a surgical tool to create a recess for an artificial joint in cartilaginous and/or bony tissue, and particularly for an artificial acetabulum in hip-joint bones, having an essentially hemispherical superficies and a hollow interior space bounded by this superficies where are formed in the superficies at least one cutting edge extending from the pole of the superficies to the equator of the superficies and having at least one cutting element (20) with a cutting surface (26) turned toward the workpiece and one cutting surface (28) turned away from the workpiece, as well as one opening in the superficies cooperating therewith to evacuate the chips toward the hollow interior space, a bearing shaft for rotation about the axis of symmetry forming the axis of rotation being fixed at the superficies and extending through the hollow interior space.
Such a surgical tool is known from WO 95/13749 disclosing a tool to form a recess in a hip-joint acetabulum which has a hemispherical tool head. This tool head has several cutting edges and openings cooperating therewith through which the material to be removed is carried away to a hollow interior space in such a way that the machining process need not be interrupted because of material having been removed.
A tool for machining bones having a hemispherical head which inclusive of the cutting edges is made of plastic material is further disclosed in EP 0 574 701 A1. A particularly economic fabrication of the tool thus becomes possible, and with it single-use applications. This document moreover discloses an embodiment of the tool having a cutting edge surrounding the shaft in a helical manner and built up from a plurality of cutting elements.
A surgical milling cutter to insert prostheses which has a hemispherical tool head and a bearing shaft to be attached with the aid of a connecting piece and wherein the tool head has a central cutting element guiding the milling cutter as well as a plurality of cutting elements distributed over the hemispherical tool head is further disclosed in EP 0 782 840 A1.
However, such tools have the disadvantage that the hemispherical tool head yields a rate of removal of material which, because of increasing cutting speed, increases toward the periphery or equator. This causes the geometric shape of the recess to depart from the hemispherical cofiguration needed for the prosthesis. As a consequence, poor fit develops between the recess and the artificial acetabulum being received therein. This leads to shorter replacement intervals for the prosthesis, implying that the patient must submit to an intervention in relatively short time intervals in order to have implanted a new prosthesis.
It is an task of the present invention, therefore, to create a surgical tool forming a recess in a cartilaginous and/or bony tissue, and particularly for an artificial acetabulum in hip-joint bones, which enables a recess having the most precise geometric shape possible to be produced.
This task is resolved with a surgical tool according to the preamble of claim 1, by having a clearance angle xcex3 of the cutting element subtended between the superficies and the cutting surface turned toward the workpiece that decreases from the pole of the superficies toward the equator of the superficies.
By having a decreasing clearance angle xcex3 the speed of removal of material decreases toward the periphery or equator of the hemisphere in such a way that the geometric shape of the recess will almost exactly correspond to the shape of the hemispherical tool. Because of the precise configuration of the hemispherical recess, a better fit between the artificial acetabulum and the recess is attained so that the lifetime of the inserted prosthesis will increase. It follows that the time intervals between interventions during which a new prosthesis must be inserted into the patient become longer.
Moreover, in the tool according to the invention, the tendency to drift is reduced as compared to conventional tools by its clearance angle decreasing toward to periphery of the tool, since compared to conventional tools, the highest cutting efficiency of the tool according to the invention is closer to the pole of the superficies. As a result, the tool according to the invention, as compared to conventional tools, is autocentering during the machining when appropriately advanced in the direction of the axis of rotation.
Further advantageous embodiments of the invention are subjects of the dependent claims.
Through an extension, fabrication of the tool is particularly simple, especially in cases where the cutting edge is composed of a plurality of cutting elements.
An extension makes it possible to further improve the cutting behavior of the tool, since the clearance angle xcex3 is continuously adapted as a function of distance from the axis of rotation.
An extension has the advantage that the tool has an essentially uniform cutting performance throughout the cutting zone since the largest depth of advance hc of the cutting edge is constant over the entire cutting periphery, in such a way that the cutting characteristics of the tool are optimized. The largest depth of advance hc, of the cutting edge is calculated from the equation: hc=2 rxcfx80 tan xcex3, from which it can be seen that there is an indirect proportionality between the distance of the cutting edge from the axis of rotation and the value of tan xcex3, the clearance angle xcex3 having values between 0xc2x0 and 90xc2x0, i.e., the clearance angle xcex3 decreases when the distance between cutting edge and axis of rotation increases.
Through an extension, the volume of material removed can be adapted as a function of the radius, in which case the clearance angle xcex3 of the cutting element and the rake angle xcex1 of the cutting element are advantageously adjusted with respect to the material being cut.
Beyond that, the extension has the advantage that the effective cutting length of the cutting edge increases, since during the removal of material, in addition to the shearing force in the direction of rotation, an additional component of the shearing force appears in an outward direction so that material can be removed with a smaller effort.
An extension has the advantage that during the cutting operation, the tool is stabilized by the fact that several cutting edges are operative at once, so that the tool will hardly depart from the desired axis of rotation. Moreover, vibrations of the tool are reduced, so that the surface quality of the recess produced is improved.
Through an extension, an advantageous stabilization and a damping of the vibrations is brought about while the tool can be fabricated at a favorable price. Moreover, at least three cutting edges are operative within the same perimeter so as to secure structural stability.
An extension guarantees, moreover, that the notches serve as chip breakers, so as to reduce the risk of seizing of the tool in the workpiece.
The extension has the advantage, moreover, that the tool is guided by a guiding element having cutting functions when in contact with the workpiece, so that vibrations between the workpiece and the tool are reduced. The tool furthermore is reinforced against mechanical deformation, in such a way that the shape of the tool is not impaired during the cutting operation.
In an extension, the tool is guided along the entire superficies during the cutting operation, since the helical guiding element scans the entire superficies during one revolution.
The extension yields an additional radial or helical cutting edge further boosting the cutting performance of the tool, since the amount of material effectively removed, and the evacuation of the chips, is further increased. It is particularly advantageous when the clearance angle xcex3 of the helical cutting edge that is subtended between the superficies and the cutting surface of the cutting element that is turned toward the workpiece decreases from the pole of the superficies toward the equator of the superficies. It is further possible to save material when fabricating the tool, as the openings between cutting edges and the helical opening overlap in such a way that the superficies of the tool essentially consists, merely of the cutting edges and the helical cutting edge.
Through an extension, the tool is additionally reinforced against mechanical deformation, since the cutting edges are multiply supported by the helix. Moreover, the points of intersection of the helix with the cutting edges serve as chip breakers.